As beginner in rockhounding, I got actually interested in the fact why a crystal grows on that certain place. I know it is important to know what kind of elements are present in the ground, the type of stone, etc, but to know the real detail about it, is hard to figure out.

So I did some research (and to make it harder, I do it in Japan), and I came on the catalogue of geological maps in Japan. I live in Nagoya, and checked for a map a little Nord of my place. I can see signs of mines, open or closed, and the type of rock.

Now here is my question: Is there anything specific I can keep in mind while reading these maps. Like places where 2 certain stones cross each other, certain rock type that has high possibility of containing crystals, etc.

Knowing why a mine is in a particular place is part of a large field of study called economic geology. If your goal is to understand why a particular mine has the suite of minerals it has, i think you should first go to mindat.org and see if you can find out if anyone has posted any info about that particular mine. From there you might find that the mine has had many different names over the years. Then just go to the internet and do google searches for info about that mine, using all the different names it has had over the years. Once you learn what type of mineral deposit the mine is exploiting, you can then go on to do google searches to find out generically why those minerals are found there. For instance, if the mine is exploiting a pegmatite, you can search for pegmatite and learn how they work and how minerals form in that geologic environment.

Good luck and enjoy. It can be fun to go down an internet rabbit hole soaking up info on something new.

Alex welcome aboard and thanks for asking a question related to the geologic context of the minerals we love, rather than just "what is it"!

I've been an economic geologist for 40 years...but I can't read Japanese, so I have to rely on some basic knowledge of geologic maps to answer your question...you have the advantage of being able to read the rock types off the map legend!

First, you are a lucky man...you live in an area of considerable geologic complexity...which is good for finding minerals! You can tell that by looking at the wide range of colors, color patterns and sharp lines across the map. Geologists represent different rock types by different colors that reflect a combination of their composition (igneous, sedimentary and metamorphic) and their age. We also talk a lot about "contacts" which is where two different rock units touch each other...lots of interesting things happen there...especially where broken rocks come together or hot rocks are next to cold ones. Your map has large areas of the same color (same unit over a large area), areas of bands and blobs of different colors (several different rock types in a small area)...some with repeating banded patterns (repetition of beds of similar rocks) and areas that look chaotic (highly deformed rocks). The latter two are more likely prospective for minerals. There are also a fair number of faults shown here...the bold black lines with rocks that don't match across them. (sometimes you can see the sense of movement by mentally restoring patterns that are offset by the faults to their original position...an easy place to do this s the big pink circular area in the lower left which has a piece sliced off and displaced by a fault). Faults are great places to look for minerals, especially where they are near intrusions...or better yet where they were followed by intrusions (see below)

So, lots to work with regionally. You can go from here to Matt's suggestion to look for the mine symbols and see if you can match these up with names on mindat (which will also have lists of the minerals known from each mine). Or, you can use this map to sharpen your focus first. A striking thing about this map is the large number of intrusive bodies it shows. Intrusions are bodies of molten magma that are injected into the crust from depth. These are the big red and pink blobs shown on the map, and the red and pink linear bodies that follow (but aren't cut by) faults. We call the blobs "stocks" and the linear bodies "dikes". Intrusive bodies are where to look for the pegmatites Matt mentioned...they form through cooling and crystallization of magma and can be found within and in the borders of the intrusions. (You mention feldspar mines...these could be pegmatites! and such mines often discard the other minerals that accompany the feldspars...like quartz or beryl) Intrusive contacts are also good places to look for metamorphic and metasomatic (skarn) mineralization, where heat and fluids from the intrusion interact with the surrounding rocks and create new minerals. An especially good place to do this is where limestone or dolomite (one of the striped colored units...you'll need to read the explanation to see which is which) is in contact with the intrusion since these rock types tend to be very reactive.

Your map also shows interesting distorted banding patterns in the sedimentary units. These rocks were laid down as layers and should be straight across the map...but they show folding, which came from tectonic pressures. The "hinges" of the folds (where they bend the most) are good places to look for minerals because the rocks there were under strain and would have cracked, creaking openings to be filled by vein minerals.

Some of the faults might also be good places to look for veins.

So, now you know some kinds of places where interesting things may have happened. The more of the favorable features that come together the better...so look for folded and faulted limestones in contact with an intrusion for example. Now check these places for the mine symbols Matt mentioned...this will confirm that mineralization occurred there [Many maps will have the chemical symbols for elements next to the mine symbols (Cu for copper, Pb for lead etc) so you know what kinds of minerals to expect.] Now, go to mindat and see what minerals are reported from these mines and look to see if there are pictures posted of specimens from the mines...people tend to post pictures of well crystallized things, so that tells you where you might have better luck...but it can be fun to be the first to find something good from an otherwise unknown area!

The more you learn of basic geology the more sense this map will make and you'll have a better sense of where and why minerals occur where they do. You might do well to focus on a certain place where you know you have permission to collect and have good reason to believe specimen quality materials may be present.

And you can also reach out to experienced collectors in your area who might be willing to help you find a place to collect and what to expect there._________________Siempre Adelante!

The field you are asking about, how to interpret the geological map is something which takes not only 5 years of University studies to get a clue about, but typically additionally 20-40 years of study to begin to understand how little you understand.

Anyhow there are many different types of deposits, which may not have been mined but which may be interesting to study in situ and which may yield collectable and interesting mineral specimens.
I can not see details on your map but for instance in the south you can see a round larger likely intrusion of igneous rock in the shape of an oval which is offset by a N-S fault. Large or small tectonic fissures may yield mineralizations. The best typically in smaller fissures or where they meet larger ones. Quartz or sulfides is one example. You may have contact metamorphoses around such an intrusion, typically where it meets a limestone you get skarns of garnet and other collectable silicates. Now the exact locations of better crystals may take some days, weeks, or months to locate and may not have economical significance but yield some nice crystals for your collection.
I have myself followed such geological hints to find rather nice specimens.
You may have zonations within an intrusion of various igneous rocks with units of higher fractionation or localized mineralization. You may have greisens or pegmatite veins within the igneous rock unit or in the surrounding rocks within some kilometer or kilometers. You may have hydrothermal veins... and on and on.

These are just the tip of the iceberg hints.

Now, mines, old mine dumps, quarries, construction sites and roadcuts in all the geological units, and their border zones should give you a hint what may be found.

When it comes to very fine crystals, remember they may occur within a cubic meter among cubic kilometers so patience is a factor and determination to research, learn, study in the field. Read everything you can on Mindat about localities in the area of interest. Now only a fraction of all interesting sites are yet on Mindat. Perhaps in the future you can help adding some when you have some in depth knowledge and have found and analyzed and understood geological context.
Good luck.

Many very successful mineral collectors have little formal training in geology so have faith! But, you might look for an introductory or adult education class in geology that you could take in your spare time. Or there are DVDs that are good for the same purpose. Not suggesting you need this, but being familiar with the terminology can make it easier to read geologic maps are recognize different rock types in the field_________________Siempre Adelante!

I concur fully with the excellent answers/suggestions already provided to you. May I also suggest that if you are really interested in learning where minerals could occur and specimens be found, it might be useful for you to purchase a general introductory text book such as Park and MacDiarmid's Ore Deposits or Alan Bateman's Economic Mineral Deposits. Yes, these text books are somewhat dated but are excellent sources of basic information describing various types of mineral deposits and why they are where they are. It is because these text books are older (published in the 1950-1960s) that they are not overly complicated and much more readable by a person without much knowledge of geology and geochemistry than text books published today. Even though you may not have any or much background in geology and related subjects, just by looking at the figures in these relatively basic textbooks and reading just the introductory paragraphs about the settings of various types of mineral deposits, you may get some ideas as to likely places to look for mineral specimens. My best wishes for finding mineral specimens you will treasure because you did, in fact, find them.

Amidst all of the great advice above, I didn't see what I would consider to be the most important--join a local club or group of mineral enthusiasts. You'll learn a lot from people with more experience who have already gone through the learning process. Local clubs often include retired geologists/mineralogists or amateurs who are experts at sniffing out mineral localities in the context of the local geology.

Everybody really thanks a lot for all the great information. The geological fact on mining crystals and interesting rocks is for me very important. All this information will get me on the right track to study more about geology.

I often ask these questions cause l am a gaijin (non japanese speaking foreigner) in Japan. That's why it is very hard for me to join or even find clubs or just information. Since mindat is mainly an english site, there is not much information about japanese mines either. But then l can consider it my task to add more information on this site.

Anyway, l'm still just a puppy among the big rockhounds, but with my (un)healthy interest in this topic l want to contribute as much as l can.

Perhaps you can find a local geologist/mineralogist or mineral collector who speaks English if you advertise somewhere or ask at a mineral museum in your area [or mineral club], if you can find a local friend to ask for you. Certainly at a University bookstore or online you can find the needed geology textbooks. Some of the books by Sinkankas are good for beginners in learning the skills of collecting and identifying what you've found. While you might not speak Japanese, the language the rocks speak is universal. Happy hunting! I think a local or not-so-local mineral museum is a must-visit, to see what might be found locally or in the areas around. More technical journal articles on minerals and geology of the area might be possible to find and are often in English or will have English abstracts and then you can look at the maps, pictures and diagrams...but that will take a university library perhaps and some work on how to research in such journals. You have you work cut out for you. In fact, most mineral collectors who are not trained geologist do not often [or ever] use geologic maps, but more on having learned what to look for with the eye of minerals on the surface and the places they come in contact or show other signs. The map, as has been expertly described above, can give you some advance clues as to where to start you looking. It takes patience unless you have a clue from someone else or an old mine. I think the old mines might be of interest. Working mines you'll need permission to enter on a day they are not working. Abandoned mines are usually just there for the pickings...and one can be surprised what the miners threw over their shoulders...but dig a little, as what was on the surface has often been picked up or weathered away - but not always. Much to learn and I think very rewarding. Old hands at this develop a 'second sense' as to where to look - along with primary knowledge from the science involved. I got hooked at age six. By age eight I was asking my parents to drive me places I had interest in and once found a 35 Kg quartz crystal lying by a pegmatite dyke. I was by then addicted - and collecting minerals,books, and love are some of the few healthy addictions ;-). One last thought. If there is a geology department at a local university, a professor or student who speaks English may be willing to help you on you way generally or locally. There must be a book with photos of Japanese minerals with locations - and a friend can perhaps help you with what is said and the locations. Lastly, likely not easy to find outside of a geology library, but surely exists, a book with English to Japanese geological terms. With a browser that does translation you should be able to locate many of these things mentioned. Partner up with an English speaking Japanese with interest in minerals is probably your best move. Not an advertisement for Betts, but looking in my web browser under Japanese Minerals I found [among others] his website with some nice minerals from there...but there are others.

I already figured out that study, contacts, and gathering info is very important. I try to do as much as l can. Geology is a small world, rockhounding even smaller. The first days l started to look for information, it looked like everything was a big secret. But things are better now. Every day (and night) l find little bits of information, most because of all good comments and hints here.

The problem with japanese people is that they are very shy and closed. So asking them something is really hard. But it's a good idea to contact a university (library).

As you are in Japan, you probably should have a look in the National Museum of Nature and Science in Tokyo which has a few good displays of the geology of Japan. Also an exhibit of a wide range of minerals found there.

I certainly will. Also here in Nagoya is a museum of science, also on the list to do. Actually many old mining companies around changed from mining to museum after the mine got depleted. Most are not so interesting, but now and then I can find some information.

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